Ternary Search Algorithm

ternary_search.cpp

@@ -0,0 +1,116 @@
+/*
+ * This is a divide and conquer algorithm.
+ * It does this by dividing the search space by 3 parts and
+ * using its property (usually monotonic property) to find
+ * the desired index.
+ * 
+ * Time Complexity : O(log3 n)
+ * Space Complexity : O(1) (without the array)
+ */
+
+#include <iostream>
+using namespace std;
+
+/*
+ * The absolutePrecision can be modified to fit preference but 
+ * it is recommended to not go lower than 10 due to errors that
+ * may occur.
+ *
+ * The value of _target should be decided or can be decided later
+ * by using the variable of the function.
+ */
+
+#define _target 10
+#define absolutePrecision 10
+#define MAX 10000000
+
+int N = 21;
+int A[MAX] = {1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,1,2,3,4,10};
+
+/*
+ * get_input function is to receive input from standard IO
+ */
+void get_input()
+{
+  // TODO: Get input from STDIO or write input to memory as done above.
+}
+
+
+/*
+ * This is the iterative method of the ternary search which returns the index of the element.
+ */
+int it_ternary_search(int left, int right, int A[],int target)
+{
+  while (1)
+    {
+      if(left<right)
+	{
+	  if(right-left < absolutePrecision)
+	    {
+	      for(int i=left;i<=right;i++)
+		if(A[i] == target) return i;
+
+	      return -1;
+	    }
+
+	  int oneThird = (left+right)/3+1;
+	  int twoThird = (left+right)*2/3+1;
+
+	  if(A[oneThird] == target) return oneThird;
+	  else if(A[twoThird] == target) return twoThird;
+
+	  else if(target > A[twoThird]) left = twoThird+1;
+	  else if(target < A[oneThird]) right = oneThird-1;
+
+	  else left = oneThird+1, right = twoThird-1;
+	}
+      else return -1;
+    }
+}
+
+/* 
+ * This is the recursive method of the ternary search which returns the index of the element.
+ */
+int rec_ternary_search(int left, int right, int A[],int target)
+{
+  if(left<right)
+    {
+      if(right-left < absolutePrecision)
+	{
+	  for(int i=left;i<=right;i++)
+	    if(A[i] == target) return i;
+
+	  return -1;
+	}
+
+      int oneThird = (left+right)/3+1;
+      int twoThird = (left+right)*2/3+1;
+
+      if(A[oneThird] == target) return oneThird;
+      if(A[twoThird] == target) return twoThird;
+
+      if(target < A[oneThird]) return rec_ternary_search(left, oneThird-1, A, target);
+      if(target > A[twoThird]) return rec_ternary_search(twoThird+1, right, A, target);
+
+      return rec_ternary_search(oneThird+1, twoThird-1, A, target);
+    }
+  else return -1;
+}
+
+/*
+ * ternary_search is a template function
+ * You could either use it_ternary_search or rec_ternary_search according to preference.
+ */
+void ternary_search(int N,int A[],int target)
+{
+  cout << it_ternary_search(0,N-1,A,target) << '\t';
+  cout << rec_ternary_search(0,N-1,A,target) << '\t';
+  cout << '\n';
+}
+
+int main()
+{
+  get_input();
+  ternary_search(N,A,_target);
+  return 0;
+}